Heretofore, as a power converter configured to convert AC power of an electric power system into DC power or an electric power conversion apparatus configured to convert DC power to AC power to drive a motor, a three-phase two-level converter or a three-phase two-level inverter has been employed. A three-phase two-level inverter is constituted of a minimum necessary number of six semiconductor switching elements required to constitute an electric power conversion apparatus configured to convert DC power into three-phase AC power, and hence downsizing and cost reduction can be realized.
On the other hand, regarding an output voltage waveform thereof, when an input DC voltage is Vdc, switching to two values +Vdc/2, and −Vdc/2 is carried out for each phase by pulse-width modulation (PWM), and a waveform is a spuriously formed AC waveform. Besides, in a high-voltage motor drive device in which high-voltage switching elements are used, and the PWM switching frequency cannot be made high, and apparatus connected to a power system of DC power transmission or the like, a filter constituted of a reactor, and capacitor is inserted in the three-phase AC output in order to reduce switching harmonics. However, in order to reduce the harmonic components flowing into the power system to a level at which other devices are not adversely affected, the capacity of the filter becomes large, and an increase in cost, and an increase in weight are brought about.
Further, in the conventional circuit form, as in the case of a modular multilevel converter shown in FIG. 6, research and development of an electric power converter in which chopper circuits are connected in a multistage form, and which can convert a high voltage equal to the voltages of the power system, and distribution system is now advanced.
When such an electric power converter is put into practical use, a transformer heavy in weight, and large in volume, and having a high proportion of the cost of the whole system is simplified, and the output voltage/current waveform is made approximate to a sinusoidal waveform by the approach toward the multilevel state, and hence it is possible to have the benefit of eliminating the harmonic filter.
In this circuit form, in order to control the voltage value of the DC capacitor serving as a constituent element of each converter unit to be constant, it is necessary in principle to make a reflux current in the DC power supply flow at all times. The three phases of power are connected to one DC power supply, and hence when DC voltage resultant values of the respective phases differ from each other even to a small extent, there is a danger that an excessive short-circuit current flows between the phases to thereby destroy the apparatus. In order to prevent such an accident from occurring, a buffer reactor is inserted in each phase to thereby limit the short-circuit current, and prevent the short-circuit current from becoming excessively large. The buffer rectors bring about an increase in size and an increase in cost of the apparatus.
As a method of solving this problem, a main circuit configuration shown in FIG. 7 is proposed. In this configuration, a single-phase three-winding transformer is provided in each of the U, V, and W phases. By using this method, it is possible to provide a small-sized electric power converter capable of outputting a voltage/current waveform with a low level of harmonics without a harmonic suppression filter while eliminating a high-cost, and large-sized reactor such as a buffer reactor.